The Hall test, invented in 1879 by E. H. Hall and improved by L. J. van der Pauw in 1958, is commonly used to measure carrier concentration and mobility in semiconductor samples. It requires four electronic contacts to be soldered to the surface of the semiconductor sample. The test is time-consuming (on the order of a few hours) and is also subject to large errors due to contamination introduced by soldering. In low-mobility samples such as n-type silicon, the contamination typically results in a 10% or more rise in carrier concentration and approximately 10% drop in mobility due to infusion of the solder's impurities into the sample. In high mobility materials such as mercury cadmium telluride (HgCdTe) and indium antimonide (InSb), soldered contacts can easily result in excess of 100% error in carrier concentration and 50% or more error in mobility. This is due to the "doping" effect of the solder's impurities into the sample at soldering temperatures. For this reason, particularly in thin films, the Hall test is destructive, usually requiring an expendable or "witness" sample that cannot be subsequently used for device manufacture.
Faraday rotation is the rotation of the plane of polarization of light as it passes through a sample in the presence of a magnetic field whose field lines are in the direction of the light. In semiconductors, Faraday rotation has two components: the plasma component which is due to free carriers and the interband component which is due to free carrier transitions across the band gap. The plasma component (.theta..sub.n) is proportional to the free carrier concentration (N), the effective mass (m.sup.*) and the wavelength of light (.lambda.) used according to ##EQU1## where q is the electron charge, c is the speed of light, .epsilon..sub.0 is the permittivity of free space, m is the electron mass (these values are constants along with .pi.), B is the magnetic field strength used, L is the thickness of the sample, and n is the refractive index of the material. By far the most popular known method of determining free carrier effective mass in semiconductors is via measurement of Faraday rotation at a single wavelength as the light travels through the semiconductors. Faraday rotation utilizing a single wavelength is useful in measuring the plasma component of the rotation in semiconductors such as silicon (Si) and gallium arsenide (GaAs) where the interband component is negligible due to their relatively wide band gaps. But in narrow band gap semiconductors such as HgCdTe and InSb where the interband component is significant, the plasma component of the Faraday rotation cannot be isolated from the interfering interband component. Indeed, until the invention of this device, measurement of electron effective mass in narrow band gap semiconductors such as HgCdTe and InSb was very limited if not impossible. HgCdTe was discovered as a promising infrared detector material in 1959. The electron effective mass is an important parameter in the design of detectors but it could not be measured in HgCdTe and other narrow band gap semiconductors such as low-doped InSb. Several calculated values of effective mass of HgCdTe were reported in the early 1980's but these were seen to be significantly inaccurate, especially at higher temperatures. Room temperature values were of particular interest but were unavailable. Effective masses of free electrons in HgCdTe and InSb were recently measured for the first time using this device.